1. Introduction
The present invention relates to radiation sensitive compositions and, more particularly, to photoimageable compositions that exhibit substantial flexibility upon curing, and processes and articles of manufacture that comprise such compositions.
2. Background Art
The need exists for new photoimageable coating compositions, particularly for electronics-related applications. For example, soldermasks, also known as solder resists, are compositions used to mask off a portion of a printed circuit board and prevent those areas from accepting solder during solder processing. See Coombs, Printed Circuits Handbook. ch. 16, McGraw Hill (3rd ed., 1988), incorporated herein by reference. Known commercially available photoimageable soldermasks form a relatively brittle coating layer upon curing. A brittle soldermask coating can pose notable problems. For example, a brittle coating may be degraded during processing and prior to application of solder, thereby resulting in complete rejection of the board or requiring the removal of the cured composition and board reprocessing.
Moreover, such brittle coatings are not useful for flexible circuit manufacture. Flexible circuits have been defined as a patterned arrangement of printed circuits and components utilizing flexible base materials with or without non-photoimageable flexible cover layers or "coats". See Coombs, Printed Circuits Handbook, ch. 35, McGraw Hill (3rd ed., 1988), incorporated herein by reference. For typical applications, only two types of cover coats are used--non-photoimageable polyimide and polyester. Other non-photoimageable cover coats also are used such as Teflon and Ultem, but their utility is typically limited to specialized applications. Current fabrication methods of a flexible circuit include prepunching the cover coats with holes for solder pads and then carefully aligning the cover coat on the base material. Current industry demands call for more aperatures in the cover coat layer, more precise registration of the aperatures with the base layer and good bonding of the cover coat to the underlying base layer. To achieve these goals, it would be desirable to have a photoimageable dielectric coating composition that upon curing exhibited sufficient flexibility to serve as a cover coat layer. By employing a photoimageable composition, aperatures could be imaged and developed in the cover coat layer.
Multichip modules have gained increased use, particularly in advanced computer systems. See Technology Update (9108, Multichip Modules and Their Impact on Board Makers," RP428-RT, BPA (Technology and Management) LTD (September 1991); and J. Balde, "Multichip Modules: The Construction in Use Today", Proceedings NEPCON-West, 965-974 (February 1990), both incorporated herein by reference. Materials of construction of multichip modules include for example aluminum nitride, silicon carbide, alumina coated copper-tungsten, silcion wafers with external mechanical support and CVD diamond coated ceramics. Manufacture of devices comprising such substrate materials will require dielectric coating compositions with improved CTE characteristics at operation temperatures (e.g., a CTE of less than 30 ppm at 200.degree. C.) and ever lower dielectric constants, including dielectric constants of less than about 3. It thus would be desirable to have coating compositions that provide such performance characteristics and could be used in MCM manufacturing processes.
Present trends in the printed wiring board industry are towards increasing the number of circuit layers in a board and decreasing the widths of formed lines and spaces. Conventional multilayer printed circuit boards comprise glass cloth reinforced copper-clad plastic substrates that range in thickness from 4-8 mils for the insulative plastic alone. After etching desired circuitry configurations on a signal layer, the board innerlayers are laminated to form a multilayer board comprising circuitry, ground plane and power plane levels. Holes are drilled through the board stack and the hole walls plated to formed conductive interconnects between the multiple board layers. This manufacturing process poses notable problems such as drill wear, smear and hole size limitations, all inherent in such a drilled board approach. Also, circuit densification is curtailed by the presence of relatively large via apertures (typically 12-20 mil in diameter) which necessitate pads that consume additional board space.
A means to avoid such problems is a multilayer circuit board manufacturing process where board layers are formed sequentially using selective plating techniques and imaging of dielectric materials to achieve fine line resolution and interconnections between circuits. Such a manufacturing process has been described in U.S. Pat. No. 4,902,610, incorporated herein by reference. It thus would be desirable to have a new photoimageable coating composition that has utility as a dielectric material in such a sequential circuit board fabrication process.